The present invention relates to optical waveguide articles having a novel optical design and to their manufacture. In particular, the present invention relates to a novel optical fiber and preform including a ring of high fluorine concentration and methods to produce the article, and to core glass compositions.
The term optical waveguide article is meant to include optical preforms (at any stage of production), optical fibers and other optical waveguides. Optical fibers usually are manufactured by first creating a glass preform. There are several methods to prepare preforms, which include modified chemical vapor deposition (MCVD), outside vapor deposition (OVD), and vapor axial deposition (VAD). The glass preform comprises a silica tube. In MCVD different layers of materials are deposited inside the tube; in OVD and VAD different layers are deposited on the outside of a mandrel. The resulting construction typically is then consolidated and collapsed to form the preform, which resembles a glass rod. The arrangement of layers in a preform generally mimics the desired arrangement of layers in the end-fiber. The preform then is suspended in a tower and heated to draw an extremely thin filament that becomes the optical fiber.
An optical waveguide usually includes a light-transmitting core and one or more claddings surrounding the core. The core and the claddings generally are made of silica glass, doped by different chemicals. The chemical composition of the different layers of an optical waveguide article affects the light-guiding properties. For certain applications, it has been found desirable to dope the core and/or the claddings with rare earth materials. However, in rare earth-doped silicates it is difficult to simultaneously achieve high rare-earth ion solubility, good optical emission efficiency (i.e. power conversion efficiency) and low background attenuation, owing to the propensity for rare-earth ions to cluster in high silica glasses.
Introduction of high concentrations of fluorine into the core glass lowers the loss and improves rare earth solubility. Fluorine is used in the core of optical fibers in which the fluorine diffuses out of the core to raise the core index or to provide optical coupling uniformity or mode field diameter conversion.
There are several methods to introduce fluorine into the core of an optical fiber: (1) chemical vapor deposition (CVD), which includes modified chemical vapor deposition (MCVD), outside vapor deposition (OVD), vapor axial deposition (VAD), and surface plasma chemical vapor deposition (SPCVD); (2) solution doping CVD-derived soot with fluoride particles or doping with a cation solution and then providing a source of fluoride (gas or HF solution); (3) sol-gel deposition of a fluoride containing core layer; (4) direct melting techniques with fluoride salts; and (5) gas phase diffusion of fluorine into the core layer before or during collapse.
Each method has drawbacks. For example, method (1), direct incorporation of fluorine by CVD methods, currently is limited to about  less than 2 wt % fluorine unless plasma CVD is used. Deposition conditions generally must be reengineered every time the relative amount of fluorine is changed. In a solution doping embodiment, soot porosity along with the doping solution concentration determine the final glass composition. Constant re-engineering is especially problematic for solution doping where the melting point and viscosity of the glass, and thus soot porosity change rapidly with fluorine concentration.
In method (2), solution doping with fluoride particles may lead to inhomogeneities from particles settling out of solution during the contact period. Exposure of a cation-doped soot to a fluoride containing solution can lead to partial removal of cations owing to resolubilization in the fluoride containing liquid. In the case that a gas is used as a fluoride source, the gas may etch the porous soot and alter the silica to metal ion ratio.
For method (3), sol-gel deposition, drawbacks include the propensity of sol-gel derived layers to crack and flake. If thin layers are used to attempt to avoid these problems, the need arises for multiple coating and drying passes.
For (4), direct melting techniques, drawbacks include the handling of hygroscopic metal salts, many of which present a contact hazard. In addition, there are difficulties uniformly coating a melt on the inside of a tube.
Finally, for method (5), gas phase reactions, the gas may etch some of the silica and change the silica to dopant ion concentration.
Fluorine (in the form of fluoride ions) has a high diffusion coefficient in oxide glasses. Fluorine will rapidly diffuse from a region of higher concentration to lower concentration. The ability of fluorine to rapidly diffuse is utilized to mode match fibers of dissimilar physical core dimensions. Fluorine diffusion out of the core into the cladding layer is used in the production of fiber optic couplers and splitters to improve the uniformity of optical coupling. Fluorine diffusion out of the core also may be used for mode field diameter conversion fiber.
Direct fluorination of the core of a fiber to provide a graded coefficient of thermal expansion (CTE) and viscosity may be beneficial to the optical properties, such as a reduction in the stimulated Brillion scattering.
Also, it is further recognized that the presence of large amounts of fluoride in oxyfluoride glasses is beneficial to prevent phase separation and clustering of rare earth, and also that clustering of fluorescing rare earth ions, such as Er3+, has deleterious effects on spectral breadth, excited-state lifetimes, amplification threshold (pump power needed to invert an optical amplifier), and power conversion efficiency of an optical amplifier. Rare-earth-doped aluminosilicate glasses have been doped with fluorine. For example, it has been reported that rare-earth-doped aluminosilicate glass doped with fluorine exhibits remarkable light emission characteristics, including high-gain amplification and broad spectral width.
Fluorine also may be doped into the cladding of optical fiber preforms. Depressed index claddings can, for example, suppress leaky mode losses in single mode fibers. Depressed index clad designs, where the index lowering dopant ions, such as F and B, are in the cladding have been used to control chromatic dispersion, for example.
Preforms may be made from fluorine-containing substrate tubes. Such tubes may be used to form silica core waveguides by diffusion of index lowering species, such as fluorine, out of the inner portion of the tube prior to collapse. In depressed index substrate tubes, there is fluorine in the substrate tube to provide favorable waveguiding properties or to diffuse out of the tube entirely to raise the local index of the innermost region.